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Thứ Sáu, 4 tháng 10, 2013

NON-CONTACT ULTRASOUND for MEDICAL IMAGING

High intensity focused ultrasound in
air may provide a means for medical and biological imaging without direct coupling
of an ultrasound probe. In this study, an approach based on highly focused
ultrasound in air is described and the feasibility of the technique is
assessed. The overall method is based on the observations that (1) ultrasound
in air has superior focusing ability and stronger nonlinear harmonic generation
as compared to tissue propagation and (2) a tightly focused field directed into
tissue causes point-like spreading that may be regarded as a source for
generalized diffraction tomography. Simulations of a spherically-curved
transducer are performed, where the transducer's radiation pattern is directed
from air into tissue. It is predicted that a focal pressure of 162 dB (2.5
kPa) is sufficient to direct ultrasound through the body, and provide a small
but measurable signal (~1 mPa) upon exit. Based on the simulations, a
20 cm diameter array consisting of 298 transducers is constructed. For
this feasibility study, a 40 kHz resonance frequency is selected based on
the commercial availability of such transducers. The array is used to focus
through water and acrylic phantoms, and the time history of the exiting signal
is evaluated. Sufficient data are acquired to demonstrate a low-resolution
tomographic reconstruction. Finally, to demonstrate the feasibility to record a
signal in vivo, a 75 mm × 55 mm section of a
human hand is imaged in a C-mode configuration.

Non-contact ultrasound

From Wikipedia, the free encyclopedia

Non-contact ultrasound (NCU) is a method of non-destructive testing where ultrasound
is generated and used to test materials without the generating sensor making
direct or indirect contact with the test material or test subject. Historically
this has been difficult to do, as a typical transducer
is very inefficient in air.[1]
Therefore most conventional ultrasound methods require the use of some type of acoustic
coupling medium in order to efficiently transmit the energy from the sensor to
the test material. Couplant materials can range from gels or jets of water to
direct solder bonds. However in non-contact ultrasound, ambient air is the only
acoustic coupling medium.An electromagnetic
acoustic transducer (EMAT), is a type of non-contact ultrasound that
generates an ultrasonic pulse which reflects off the sample and induces an
electric current in the receiver. This is interpreted by software and provides
clues about the internal structure of the sample such as cracks or faults. [2]Research is continuing to improve
traditional transducers by applying different plastics, elastomers,
and other materials. The sensitivity of these devices continues to improve; a
newly developed piezoelectric transducer can
produce frequencies in the MHz that can easily propagate through even high
acoustic impedance materials such as steel and dense ceramics.[1]Non-contact ultrasound allows
some materials to be inspected which otherwise can’t be inspected due to fear
of contamination from couplants or water. In general non-contact ultrasound
would facilitate testing of materials or components that are continuously
rolled on a production line, in extremely hot environments, coated, oxidized,
or otherwise difficult to physically contact. Methods for potential medical use
are also being investigated[3]Laser
ultrasonics is another method of non-contact ultrasound.